The invention is based on a combined service brake and spring-loaded brake cylinder having a pressure-loaded diaphragm, which is arranged in a housing of the service brake cylinder and which delimits, on one side, a service brake chamber which can be acted on with service brake pressure and, on the other side, a chamber which holds a restoring spring. A spring brake piston is arranged in a housing of the spring brake cylinder and can be actuated by a preloaded spring. The spring brake piston delimits, on one side, a spring brake chamber and, on the other side, a spring chamber which holds a preloaded spring and which has a piston rod which carries a ventilation valve which produces or blocks a flow connection between the spring chamber.
A combined service brake and spring-loaded brake cylinder of the above-described type is known, for example, from DE 40 11 739 A1. There, the ventilation (air-bleed) valve is arranged at the end side in the hollow piston tube of the spring brake piston, which can project into the service brake chamber depending on the operating state. The air-bleed valve has the task of dissipating the overpressure generated, when the parking brake is released, by the retraction of the parking brake piston and the resulting reduced volume of the spring chamber, by virtue of the valve being switched into its open position and producing a flow connection between the spring chamber and the service brake chamber. When starting a vehicle on a planar (or flat) underlying surface, the service brake chamber is deaerated and connected to a deaeration system of a pressure regulating module, since a service braking process is not necessary after the release of the parking brake. It is then possible for at least part of the excess air volume in the spring chamber to flow out, which accordingly passes from there not directly into the atmosphere, for example via a valve arranged in the wall of the spring chamber, but rather via the aerating and deaerating path of the service brake chamber. In this context, the term internal deaeration or internal ventilation is therefore also used.
When starting on an uphill underlying surface, it is however necessary, when the parking brake is first applied, to at least briefly also apply the service brake before releasing the parking brake in order to prevent the vehicle from rolling backward when starting. In this case, the service brake chamber is aerated. In the event of a sufficiently high braking demand by the driver, the service brake pressure, which prevails in the service brake chamber and simultaneously at one side of the piston, seeks to hold the piston against the valve seat, and thereby to hold the deaeration valve closed, counter to the action of the pressure building up in the spring chamber. However, if the service brake pressure and/or the service brake pressure gradient are below certain threshold values on account of a correspondingly low service braking demand by the driver, then the service brake pressure prevailing at one side of the piston is not sufficient to hold the deaeration valve closed. Compressed air then flows from the service brake chamber via the open deaeration valve into the spring chamber. From there, the compressed air escapes via the piston seal and the housing seal into the atmosphere, which first generates disturbing noises. Secondly, the air volume which flows out via the deaeration valve is no longer available for building up the service brake force.
The present invention is based on the object of further developing a combined service brake and spring-loaded brake cylinder of the type mentioned above such that the above-specified disadvantages are avoided.
According to the invention, the air-bleed valve of the combined service brake and spring-loaded brake cylinder has the following features:
a) a piston which carries a valve body, which piston is guided in an axially movable manner in a cylinder, with at least one first pressure spring which is supported on the piston loading the valve body against a first valve seat on the piston;
b) the piston is loaded by at least one second pressure spring, which is supported against the piston rod, in such that the valve body is forced in the direction of a second valve seat on the piston rod and away from the first valve seat;
c) a first piston face of the piston is loaded by the pressure in the spring chamber in a direction which lifts the valve body from the second valve seat, and a second piston face is loaded by the pressure in the service brake chamber in a direction which forces the valve body against the second valve seat and lifts the valve body up from the first valve seat;
d) a flow connection between the service brake chamber and the spring chamber being produced when the valve body is lifted up from the first valve seat and/or when the valve body is lifted up from the second valve seat.
By way of the measures, it is ensured that the air-bleed valve remains closed if the service brake is simultaneously applied when the parking brake is applied. It is, therefore, no longer possible for compressed air to flow over from the service brake chamber via the air-bleed valve into the spring chamber and to escape from there into the atmosphere, as a result of which disturbing noises are prevented.
Advantageous refinements of and improvements to the invention specified in the independent claims are possible by means of the measures listed in the claims.
More precise details can be gathered from the following description of exemplary embodiments.
An exemplary embodiment of the invention is illustrated below and explained in more detail in the following description.